Unfortunately I have to start with a disclaimer: I am not a medical professional. What I present here is my opinion from my experiences. If you have any questions or health concerns you should seek the advice of a qualified health care professional.
I live in east Texas and we have a unique problem in that when it is hot, it is really hot. In the summer however, people really do die from the heat, and you can get heat exhaustion or worse without really any physical activity. So how so you stay cool while out observing?
The first thing is shade assuming you are doing solar observing during the day. This is the only difference between day and night observing. You absolutely need to stay in the shade. Most major retailers sell four post canopies for very little such as <<insert amazon link here>>. These are worth every cent you spend on them. I also recommend you use as little sunblock as possible as it seems to make it harder for my natural sweat to do its job cooling me off.
If you absolutely have to be out in the sun, wear a light colored breathable hat. I like the golf hats made by Nike, Under Armor and others. The wide brim hats with mesh around the top are also a nice choice.
I also find that a quality fabric shirt such as a Nike or Under Armor running type shirt that fits a little snug (not skin tight, not really form fitting, just not really loose) works far better than a loose or inexpensive shirt. I am not big on name brand clothes except for certain items, and shirts to keep me cool is one of them. I have tried several less expensive shirts from major sporting goods stores which claim to be excellent for keeping you cool and they all perform substantially poorer than the Nikes I generally wear. If you want to save some money you can find the Nike running shirts cheap at Nike outlet stores. They do not have to be running shirts, they just need to be the high end Nike shirts designed specifically to keep you as cool as possible. Remember that if you are out in the day, the color of the shirt is important as well, lighter colors reflect heat and dark colors absorb them. Wearing black is not a really good idea for solar observing in the heat of the summer.
I have even tried wearing both shirts under my motorcycle riding jacket (a mesh jacket) and got exactly the same results. The Nike shirt felt substantially cooler than the others. The trick is that the material in the shirt needs to wick sweat (take the sweat off your skin and deposit it on top of the fabric). When this works well it is amazing. When it does not (think of a cotton t-shirt soaked with sweat) you can suffer a real health problem from overheating.
Next is moving air. Your body keeps cool by sweating. This moisture is removed from your skin (or shirt) by wind moving across your skin (or shirt) and evaporating that moisture. This removes heat and makes you feel cooler. No wind means far less cooling. I have found a small fan that moves quite a bit of air and does not make too much noise. This little fan on low has substantially increased my enjoyment of astrophotography during the summer.
No amount of cooling is enough if you do not stay hydrated. Take plenty of water. A typical recommendation is to drink six or eight eight ounce glasses of water a day but this is for normal activity without a lot of sweating. All that sweat has to come from somewhere, and it all has to be replaced. When I am out at night in the summer where it can be up to 100F at sunset and not drop below 85F before sunrise I try to drink one 16.9FL OZ bottle of water an hour. Normally I carry six bottles in a cooler and a few more in the back of the car and I normally come back with an empty or almost empty cooler. As the night progresses and gets cooler I tend to sweat less and drink less.
You certainly can drink things other than water if you like, just remember that many of them may not hydrate you as well as water and some, such as alcoholic beverages, can actually dehydrate you. Be careful of the warning signs such as not feeling hot, feeling cold, not being thirsty at all, not going to the bathroom for an extended period, or feeling dizzy. If you have any of these symptoms, seek assistance immediately.
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